Epoxides are highly reactive chemical compounds which as a result of their reactivity, can be used in a wide variety of application. Epoxidation is a second-order and is highly exothermic reaction with heat of reaction (ca. 250 kJ/mol); therefore, care must be taken at all times to ensure safe operation. Electron-donating groups, e.g., alkyl groups at the double bond carbon atoms, enhance the reaction rate while electron-withdrawing groups have the opposite effect and may sometimes stop the reaction entirely. For example, it has been reported (D. Swern, J. Am. Chem. Soc. 69 (1947) 1692) that 2-Butene reacts considerably faster than propene, whereas allyl chloride reacts slower.
The preparation of epoxides by the oxidation of alkenes is a technically important process of economical significance. Preferably epoxides are formed by the reaction of an alkene with an oxidizing agent in the presence of a catalyst. Various oxidizing agents such as commercial bleach, organic hydroperoxides, organic per acids, iodosyl arines, oxones, molecular oxygen (in the form of pure oxygen or atmospheric oxygen) and hydrogen peroxide have been used to prepare a variety of alkene epoxides.
Hydrogen peroxide is a high oxygen content, environmentally friendly oxidant for which water is the sole by-product in heterolytic oxidation, but it is a slow oxidant in absence of activation due to the poor-leaving tendency of the hydroxide ion. (G. Strukul, Catalytic Oxidation with Hydrogen Peroxide as oxidant: Kluwer: Dordrecht, 1992 and J. O. Edwards, In Peroxide Reaction Mechanism; O. J. Edward, Ed. Interscience: New York, 1962; pp, 67). Transition metal salts or complexes have been used as catalyst for alkene epoxidation with aqueous H2O2 (E. N. Jacobsen, In Comprehensive Organometallic Chemistry II; E. W. Abel, F. G. Stone, E. Wilkinson, Eds. Pergamon: New York, 1995. Vol. 12 p. 1097; H. R. Tetzlaff, J. H. Espenson, Inorg. Chem. 38 (1999) 881). Other methods for activation of H2O2 include forming reactive peroxy acids from carboxylic acids (D. Swern, In Organic peroxides; D. Swern Eds. Wiley Interscience, New York 1971 Vol. 2 p. 355) forming peroxycarboximidic acid from acetonitrile (G. B. Payne P. H. Deming, P. H. William, J. Org. Chem. 26 (1961) 659) generation of peroxyurea (G. Majetich, R. Hicks, Synlett. (1996) 694), or using perborate or sodium percarbonate in strongly basic solution (A. McKillop, W. R. Sanderson, Terahedron, 51 (1995) 6145). A method for activating hydrogen peroxide with bicarbonate ion in alcohol/water solvents was described by R. S. Drago et al. in Proceeding of 1997 ERDEC scientific Conference on Chemical and Biological Defense Research and D. E. Richardson et al. in Proceeding of 1998 and ERDEC scientific Conference on Chemical and Biological Defense Research, ERDEC, 1999. In the bicarbonate-activated peroxide system, the active oxidant peroxymonocarbonate ion, HCO4− is presumably produced via the perhydration of CO2 (D. E. Richardson et al., J. Am. Chem. Soc., 122 (2000) 1729). Peroxymonocarbonate is an anionic peracid and is a potent oxidant in aqueous solution. Similarly nitrites have also been shown to activate hydrogen peroxide via in-situ production of potent epoxidising reagent-peroxyimidic acids in alkaline media (in general known as Payne system; G. B. Payne et al., J. Org. Chem. 26 (1961) 659; G. B. Payne, Tetrahedron 18 (1962) 763).
Reference may be made to A. Wurtz, in Ann., 110 (1859) 125 which discloses an industrial process for epoxidising double bond via the chlorohydrin, which uses chlorine as the oxidizing agent (in situ generation of commercial bleach). Disadvantages of the process are (i) there is a simultaneous production of calcium chloride as a by-product of the dehydrochlorination of the chlorohydrin, which has low economic value; (ii) this process generates chlorides of sodium and calcium as inorganic effluent in excess (5–6 equivalent more than the product); (iii) the process is not ecofriendly because of the use of chlorine.
D. Swern in “Organic Peroxy Acids as Oxidizing Agent”: D. Swem in Epoxidation, “Organic Peroxides,” 2, 5, Wiley-Interscience, New York 1971, pp. 355 described epoxidation of long-chain alkenes including vegetable oils (e.g., soya bean oil), polybutadiene, natural and synthetic rubbers and polyesters with in situ formation of performic acid or peracetic acid as oxidant. The major drawbacks in this system are (i) the peracids used are under anhydrous conditions and are in high concentrations. Under these conditions these per acids (especially lower alkyl per acids) are highly explosive; (ii) under aqueous condition, epoxides are readily hydrolyzed unless the medium is appropriately buffered; (iii) the cost of per acid is high thus it adversely affects the economics of the process.
D. W. Leyshon et al. in U.S. Pat. No. 6,583,300 (2003) discloses a process for the production of propene oxide by the reaction of propene with a hydroperoxide in presence of titanium containing molecular sieve as catalyst at 63° C. and 1000 psig pressure wherein reaction effluent comprises by weight 58% propene, 4.6% propene oxide, 10.8% methyl benzyl alcohol, 18.2% ethyl benzene, 8.4% others. The process has the following disadvantages. (i) it produces a low-cost alcohol as a by-product in an amount chemically equivalent to the epoxidised compound formed; ii) selectivity of the process is poor as it lead to the formation of unidentified products to the tune of nearly 18% by weight; iii) the catalyst deactivates after first run and need to regenerated; iv) the method is not suitable for higher and aromatic alkenes.
J. R. Monnier, et al. in U.S. Pat. No. 5,145,968, (1992) has disclosed selective monoepoxidation of styrene and styrene analogs with molecular oxygen (0.01–30 mol along with a diluent gas helium) in the presence of a silver-containing catalyst comprising 2 to 20 weight % silver, 0.01 to 2 weight % of an alkali metal nitrate or chloride as catalyst promoter on alumina support. Epoxidation reaction was conducted at a reaction pressure of 1–30 atmosphere over a temperature range of 100°–325° C., wherein conversions to the product epoxide was in the range of 5–60% with selectivity to styrene oxide was of 50–78 mol %. However, this process has following disadvantages (i) operating temperatures are higher at which alkene and oxygen mixture is a potential explosive; (ii) Conversions and selectivities are moderate which limits its scope for commercial application; (iii) utilizes expensive helium gas as diluent for maintaining oxygen concentration.
T. Mukaiyama in Bull. Chem. Soc. Jpn. 68 (1995) 17 and G. Pozzi in Chem. Commun. (1997) 69 reported the use of molecular oxygen (in the form of pure oxygen or atmospheric oxygen) as the oxidant for achieving high epoxide yield of epoxycyclohexane, in presence of an aldehyde as additive in methylene chloride as solvent, using iron/copper powder and catalytic amounts of acetic acid as catalyst. In continuation of this study, S-I. Murahashi et al. in EP 0 540 009 (1993) disclosed that a catalyst can even be completely dispensed with under dilute (using excess of methylene chloride) condition. Both of these process have the following disadvantages (i) the process uses large amounts of methylene chloride. as solvent, which is ecologically and toxicologically dangerous; (ii) other solvents such as toluene cannot replace methylene chloride and results in lower yields and side reactions through oxidation of the solvent; (iii) many conventional organic solvents form explosive mixture with molecular oxygen, which greatly limits its application in industry; (iv) in the process of oxidation of the aldehyde additive gets converted into corresponding acid, which is not desirable as far as process economy is concerned.
G. B. Payne et al. in U.S. Pat. No. 3,053,856 disclosed the use of hydrogen peroxide as an oxidizing agent in the presence of a catalyst such as tungstic acid or in the presence of an organic nitrile. But these two methods have drawbacks (i) in the case of tungstic acid the product epoxide is hydrolyzed to the corresponding glycol under the reaction condition; (ii) in the case of organic nitrile an equivalent quantity of the corresponding amide is generated along with the product epoxide; (iii) the amide generated is low cost besides it is required to be separated from the product epoxide by way of distillation process which further adds to the cost of the process, hence industrially undesirable.
M. Taramasso et al. in U.S. Pat. No. 4,410,501, (1983), G. Bellussi et al. in U.S. Pat. No. 4,701,428, (1987) and C. Neri et al. in U.S. Pat. No. 4,833,260, (1989) disclosed that titanium silicalites are effective catalysts for the epoxidation of olefinic compounds with hydrogen peroxide as oxidant in the presence or in the absence of solvents. In these cases the epoxidation is effected in a protic medium such as, an alcohol or water where alcohol is considered as a co-catalyst. However, these processes suffer following disadvantages i) the catalyst requires treatment with a neutralizing agent for suppressing the superficial acid sites of the catalyst, responsible for the formation of these undesired byproducts. In doing so inorganic salts are generated that are environmentally not desirable; ii) the small pore size of the catalyst titanium silicalites (5.6×5.3 Å) limits its application to the smaller alkenes; iii) epoxidation of a wide range of bulkier alkenes cannot be epoxidated with these catalysts since alkenes cannot reach the active sites. S. Enomoto, et al. in U.S. Pat. No. 5,041,569 (1991) and K. Nishibe et al. in U.S. Pat. No. 5,155,241 (1992) have disclosed the preparation of styrene oxide by reacting styrene and hydrogen peroxide in heterogeneous system in the presence of a bis (tri-n-alkyltinoxy)molybdic acid with an amine and an inorganic anion respectively using 60% hydrogen peroxide as a source of oxygen which took 24 hours to give 77–82% yield with 90% epoxide selectivity at 24° C. in presence of a water insoluble solvent like chloroform, dichloroethane, benzene and acetonitrile. The drawbacks of this system are i) it requires chlorinated and other hazardous solvents; ii) the yields and selectivity are on the lower side and takes longer time to achieve such conversions; iii) it requires highly explosive concentration of hydrogen peroxide (60%) to achieve above-mentioned conversions, which is not favorable for its application in industry.
B. S. Lane et al. J. Am. Chem. Soc., 123 (2001) 2933 described a method for activating buffered hydrogen peroxide (10 equivalent) with bicarbonate ion in either alcohol/water or dimethyl formamide/water in 1:1.4 ratio as solvents where active peroxymonocarbonate ion, HCO4− is presumably produced via the perhydration of CO2. Using this combination it has been reported to give a conversion of 93% for styrene to styrene oxide in 24 h. The system has following disadvantages i) amount of buffered H2O2 (10 equivalent) is appreciably large to obtain the high conversions thus oxygen atom efficiency for hydrogen peroxide is poor and require to handle very large volumes making the process not viable at commercial level; ii) it takes extended time period (16 h) to add the buffered H2O2 as the reaction is highly exothermic under these reaction conditions.
G. Majetich et al. in Synlett (1996) 649 have described acidic/base carbodiimide-promoted epoxidation of 3-phenyl 1-propene, cyclic and long chain alkenes, wherein carbodiimide in presence of hyrogen peroxide generates in situ peroxyisourea as the oxidant. The yields were found between 38–71%. This process though attractive suffers following disadvantages (i) the olefin should be soluble in alcoholic solvent, thus limits the scope of the method for other alcohol insoluble alkenes; (ii) the carbodiimde used was dicyclohexylcarbodiimide and is expensive while the equivalent amount of the urea generated at the end of the reaction is of low value; (iii) efficiency of oxygen atom utilization per mole of substrate is very poor and requires large volumes (10 fold excess) of oxidant. Hence, the possibility of scaling up such systems is difficult for industrial applications.